MCB Lecture 41 Beta-Catenin, Ion Channel receptors, GPCRs Flashcards Preview

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Flashcards in MCB Lecture 41 Beta-Catenin, Ion Channel receptors, GPCRs Deck (37)

What is an example of a pathway that relies on the proteolysis of regulatory proteins?

The Wnt pathway


What is the importance of the beta catenin pathway?

Beta catenin is normally degraded and gene expression is turned off.

However, mutations in APC, or binding of Wnt to the receptor deactivates the beta catenin destruction complex. Beta catenin levels build up and gene transcription is turned on


How does the Wnt pathway lead to cancer?

Over expression of the Wnt pathway leads to too much gene expression, and over proliferation of cells


Describe how beta catenin levels can rise in the cell cytosol

1. Mutation in APC, beta catenin is not targeted for degredation

2. Constitutively active receptor. The pathway is always on


What is the ligand in the Wnt pathway?



What is the receptor in the Wnt pathway?



Describe the signal-transduction of the Wnt pathway

1. Wnt ligand binds to the Frizzled receptor
2. Dishevelled protein is activated
3. The kinases in the beta catenin destruction complex (GSK3B and CK1) are inactivated, so they cannot target beta catenin for degredation
4. Beta catenin levels build up in the cytosol
5. Beta catenin enters the nucleus, kicks off Groucho and binds to coactivators
6. Gene transcription occurs (Wnt genes transcribed)


Describe the structure and function of the beta-catenin destruction complex



What is Groucho, and how is it involved in the Wnt pathway?

It is a protein bound to coactivators on the Wnt genes, preventing transcription

Beta catenin kicks it off so that transcription of the Wnt genes can occur


Describe how beta catenin is normally broken down

It is bound by the Beta catenin destruction complex
It is phosphorylated by GSK3B and CK1
Once phosphorylated, it is targeted by the E3 ubiquitin Ligase complex
Ubiquitin is added
It is target to a proteosome which degrades it


Which two proteins phosphorylate beta catenin for targeting by E3 ubiquitin Ligase complex?



Describe how beta catenin mutations affect skin cell differentiation

When there are mutations in beta catenin, it cannot enter the nucleus and turn on gene transcription

As a result, In these tissues, there are no stem cells that can produce hair follicles.

The rat is hairless


Describe how different mutations in beta catenin can affect the cells in the eye

WT: epithelial and fibre cells
B catenin KO: no epithelial cells
APC KO: over proliferation of epithelial cells


Describe how beta catenin mutations can affect the Adherens junctions in eye cells

WT: Adherens junctions in the epithelial layer of lens
B Cat KO: no Adherens junctions
APC KO: too many Adherens junctions


What is APC, and what do mutations in APC cause?

It is a scaffold protein in the beta catenin destruction complex

Mutations in it cause over proliferation of cells (cancer), because there is nothing to degrade beta catenin. Levels of beta catenin rise


Wnt signals regulate ... and ...

Thus, mutation causes ...

Myc and cyclin D

Over proliferation


How do colon polyps form?

There are mutations in APC

The beta catenin destruction complex is no longer active, and its levels build up and there is too much transcription of the Wnt genes. There is over proliferation of cells


Describe the structure and function of ion channel receptors.

Where are they commonly found, and what do they do here?

These are proteins in the membrane that form a channel large enough to allow ions to pass through when a ligand binds

They are commonly found in neurons, and they are involved with changing the polarity across membranes


Describe the main players in a GPCR signal transduction pathway

G protein
Target protein


Describe the structure of a GPCR

7 pass transmembrane protein


Describe the structure of a G protein

Three subunits: alpha beta gamma

Alpha and gamma are membrane bound


Describe the differences in G protein suture when it is active and inactive

Active: GTP bound, alpha subunit dissociated
Inactive: GDP bound


Describe the signal transduction pathway in GPCRs

1. Ligand binds
2. Conformational change in GPCR
3. G protein is activated: GPCR acts like a GEF: alpha subunit has GDP replaced with GTP
4. Alpha subunit dissociates and moves to a target protein in the membrane
5. Alpha activates the target protein
6. Alpha's GTPase function is now activated, and GTP is hydrolysed
7. Alpha is inactive, and binds back to beta and gamma subunits


How does the alpha subunit of the G protein replace GDP with GTP?

The GPCR acts like a GEF


What causes activation of the Target protein?

The association with the activated alpha subunit


What causes the hydrolysis of GTP on the alpha subunit?

1. Binding to the target protein
2. RGS


What is RGS?

Regulator of G protein Signalling

It activates the GTPase activity of the alpha subunit: GTP is hydrolysed and the alpha subunit is deactivated


What is a common target protein?

Adenyl cyclase


Describe the pathway that results when adenyl cyclase is activated

1. Adenyl cyclase is activated by alpha subunit of G protein
2. Adenyl cycle catalyses the conversion of ATP to cAMP
3. Much cAMP is produced (amplification)
4. cAMP activated PKA
5. PKA moves to the nucleus and activates the CREB portein
6. CREB binds to CBP
7. Gene transcription is activated


What is CREB? How does it function?

It is a protein that is activated by PKA that then goes on to activate gene transcription


What causes Albright Hereditary Osteodystrophy?
What is it also known as?

What are the clinical features?

This is a mutation in G proteins.

It causes resistance to parathyroid hormones

Aka Pseudohypoparathyroidism

Short stature
Bone malformation: short bones


Describe how the beta-gamma subunits are involved in signalling

1. Once the ligand binds to the GPCR, the beta and gamma subunits are also activated.
2. These subunits move to an ion channel in the membrane and cause it to open
3. The K channel opens, and potassium moves down its electrochemical gradient, out of the cell


Describe how photo transduction is an example of GPCRs

Rhodopsin and the co formation all change In Retinal is akin to the ligand binding

A G protein is activated (transducin)

A target protein is then activated by the alpha subunits (cGMP phosphodiesterase)


Which molecule is responsible for stem cell differentiation in the skin of mice?

Beta catenin


What features are reliant on beta catenin in the lens?

Epithelial cell layer
Adherens junctions

Loss of function: no Adherens, no epithelium
Gain of function: too much epithelium, too much adherens


Is APC a tumour supressor or a proto oncogene?

It is a tumour suppressor


Describe the important process in bone that relies on G protein coupled receptors

Bone resorption relies on a GPCR and a parathyroid hormone

Mutations in the G protein lead to Pseudohypoparathyroidism / Albright hereditary osteodystrophy

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